1. Field of the Invention
This invention relates generally to laser marking techniques and, more specifically, to an apparatus and method for marking the surface of a bare semiconductor device, comprising one or more dice, using a laser without damage to the circuitry or components of the semiconductor device.
2. State of the Art
Since the first semiconductor devices became commercially available, manufacturers have found it necessary to mark each semiconductor chip or assembly of semiconductor chips (bare die or package) with the company name, a part or serial number, or other information such as lot number or semiconductor die location. Conventional marking methods utilize a mechanical device to transfer ink contained in an ink pad to the surface of a stamp. An individual semiconductor chip is then stamped, and the automated process is repeated for subsequent semiconductor chips.
Because of its mechanical nature and the drying time associated with ink, an ink stamping process is relatively slow. Moreover, if the mark is accidentally touched prior to complete drying, the mark will smudge. In semiconductor chip manufacturing processes using such an ink stamping method, the ink marking operation may have to be included at a relatively early stage of production (if the semiconductor die itself is to be marked) or just after post-encapsulation processing (if the package is to be marked) to allow for drying time without affecting the production rate. Such early marking may result, however, in marking defective semiconductor chips that never make it completely through the manufacturing process.
Another problem associated with ink stamping methods is that the quality of ink stamped marks may substantially vary over time. This variation may be dependent upon the quantity of ink applied, ambient temperature and humidity, and/or the condition of the surface of the stamp. In any event, the consistency of a stamped mark may vary widely from semiconductor chip to semiconductor chip.
As a result of the deficiencies associated with ink stamping, it has become increasingly popular to use a laser beam to mark the surface of a semiconductor chip. Unlike ink stamping, laser marking is very fast, requires no curing time, has a consistently high quality, and can take place at the end of the manufacturing process so that only known good semiconductor chips (dice) are marked.
Various machines and methods have been developed for marking a semiconductor chip with a laser. As illustrated in U.S. Pat. No. 5,357,077 to Tsuruta, U.S. Pat. No. 5,329,090 to Woelki et al., U.S. Pat. No. 4,945,204 to Nakamura et al., U.S. Pat. No. 4,638,144 to Latta, Jr., U.S. Pat. No. 4,585,931 to Duncan et al., and U.S. Pat. No. 4,375,025 to Carlson, a semiconductor device is placed in a position, where a laser beam, usually produced by a carbon dioxide, Nd:YAG, or Nd:YLF laser, inscribes various characters or other information on a surface of the semiconductor device. Basically, the laser beam erodes or causes a transformation or change in the reflectivity of a portion of the surface of the semiconductor chip such that a different reflectivity from the rest of the semiconductor chip surface is formed. By holding the semiconductor chip at a proper angle to a light source, the information inscribed on the semiconductor chip by the laser can be read.
Various materials are known in the art that are laser reactive (e.g., capable of changing color when contacted by a laser beam). As described in U.S. Pat. No. 4,861,620 to Azuma et al., U.S. Pat. No. 4,753,863 to Spanjer, and U.S. Pat. No. 4,707,722 to Folk et al., the part or component may be partially comprised of the laser markable material or have a coating of the material on the surface of the part or component to be marked.
Using a laser to mark a semiconductor chip is a fast and economical means of marking. There are, however, certain disadvantages associated with state-of-the-art laser marking techniques that merely erode or change the reflectivity of a portion of the surface to achieve the desired mark in comparison to ink stamping. For example, ink stamping provides a clearly visible image on the surface of a semiconductor chip at nearly every angle of incidence to a light source. A mark made in a surface by a laser, on the other hand, may only be visible at select angles of incidence to a light source. Further, oils or other contaminants deposited on the semiconductor chip surface subsequent to marking may blur or even obscure the mark. Additionally, because the laser erodes or changes the reflectivity of a portion of the surface of the work piece, for bare semiconductor die marking, the energy of the laser beam and its effect on the semiconductor chip may damage the internal circuitry or components of the semiconductor chip directly or by increasing internal semiconductor chip temperature beyond acceptable limits. Additionally, for bare semiconductor chip marking the substrate upon which the bare semiconductor chip is formed may allow the transmission of the light from the laser therethrough, causing damage to the circuitry of the semiconductor chip or changes in the materials used in the components of the semiconductor chip. Thus, it would be advantageous to provide a marking technique that combines the speed and precision of laser marking without any substantial curing or drying time. Moreover, it would be advantageous to develop a method and apparatus for marking the surface of a semiconductor chip that does not harm the circuitry, components, or materials therein.
According to the present invention, a laser marking apparatus and method are disclosed wherein an object is subjected to a laser beam for marking purposes.
In one aspect of the invention, the laser is selected to have a light frequency source which is not transmissible through the substrate, i.e., the substrate is opaque to the light frequency source of the laser, or to substantially heat the substrate in bulk to thereby affect the delicate internal circuitry of the semiconductor chip or expose such circuitry to such energy source and/or potentially damaging heat.
In another aspect of the invention, subsequent to or while being marked, the semiconductor chip is subjected to a jet of coolant to rapidly cool the markings and/or prevent or reduce the potential for heat damage to the semiconductor chip. The coolant may be in a liquid, gas, or solid state. In this manner, any residual heat contained in the substrate material or present in the semiconductor chip may be rapidly dissipated. The semiconductor chip is cooled before exiting the marking apparatus.
In another more particular aspect of the invention, the laser marking apparatus is computer controlled. In addition to controlling the laser beam, chip location, and other process parameters, the central processing unit (CPU) may control the quality of markings. If so, the marked chips may be subjected to a camera which feeds an image of each chip to the CPU. The CPU compares the pixels of the captured image to a given resolution standard. If the marking is of a sufficiently high quality, the chips are automatically accepted. If not, the chips are automatically rejected for rework and remarking.